Cyclin-dependent kinases (cdks) regulate cell cycle progression and RNA transcription. This work will focus on the interaction of cdks with DNA damage pathways. Cdks are major targets of ATM (ataxia-telangiectasia mutated) and ATR (ATM and Rad3-related) checkpoint cascades in response to DNA damage, and their inhibition promotes cell cycle arrest and repair. However, accumulating evidence suggests that cdks regulate many upstream elements of the DNA damage response, prior to their ultimate downregulation. Preliminary data indicate that cdk1 participates in BRCA1-dependent S phase checkpoint control following DNA damage. After shRNA-mediated cdk1 depletion or small molecule-mediated cdk1 inhibition, checkpoint control is ineffective, so that cancer cells are sensitized to DNA damaging treatments, including cisplatin, y-irradiation and nucleoside analogs. In the first specific aim, the role of cdk1-mediated phosphorylation of BRCA1 at S1497 will be assessed to determine if this event is necessary for checkpoint control in lung and breast cancer cells. Since cdk1 depletion compromises BRCA1 function, whether cdk1 participates in homologous recombination repair or whether its depletion sensitizes cells to PARP-1 inhibition will also be studied. The selective sensitization of cancer cells to DNA damage by cdk1 depletion will be investigated using non-transformed cell lines and paired transformed derivatives. The sensitivity of cdk1-depleted cells to both cisplatin and PARP-1 inhibition will also be confirmed in vivo using xenograft models. Finally, RNA interference screens will be used to define novel targets in DNA damage and other pathways, depletion of which may augment the response to cdk1 depletion or inhibition. In order to derive results relevant to cdk inhibitors in clinical use, which inhibit multiple cdk family members, the second specific aim will focus on cells in which there is combined depletion of cdk2, cdk1 and cdk9. Depletion or inhibition of this subset of cdks induces apoptosis. The cdk2/1-dependent events leading to cell death will be characterized, including S-phase slowing, depletion of checkpoint kinase 1, induction of a DNA damage response, and impaired checkpoint control facilitating endoreduplication and re-entrance into S phase, where cells are the most vulnerable. Additionally, the role of cdk9-mediated transcriptional cdk inhibition in modulating the apoptotic threshold will be investigated. The interaction of inhibitors of cdk2, cdk1 and cdk9 with DNA damaging agents and PARP-1 inhibitors will also be assessed. RNA interference screens will be used to interrogate the intersection of combined cdk depletion or inhibition with novel targets in DNA damage and other pathways. In the third specific aim, pharmacodynamic endpoints will be incorporated into relevant clinical trials. These include a Phase 1 trial of SCH727965, an inhibitor of cdks 2, 1, and 9 with nanomolar potency, as well as a combination trial of the nucleoside analog sapacitabine with the cdk inhibitor seliciclib, the latter expected to augment the DNA damage response.